Among thermoplastic resins, polyester resin is superior in physical properties such as mechanical properties and heat resistance, and is used in electric component parts, business machine component parts and so froth. In recent years, however, the polyester resin is required to be further improved in physical properties. Specifically, a laminar clay mineral, an impact modifier, etc. are added to a thermoplastic resin and melt-kneaded by means of a kneading machine, to thereby attempt to improve physical properties. Researches are energetically made also on a composite technique for resins of vegetable origin, such as polylactic acid.
As for the composite technique, as disclosed in Japanese Patent Laid-open Application No. 2003-286396, it is attempted that a polymer of a multi-layer structure is added to polylactic acid to improve impact resistance. It is also disclosed in Japanese Patent Laid-open Application No. 2005-29759 that an unmodified natural rubber and an acryl-modified natural rubber are added to polylactic acid to improve impact resistance. However, resin compositions disclosed in these publications are improved in impact resistance when compared with those to which the stated material(s) has or have not been added, but do not have strength enough to be used as structural component parts.